This invention relates to electronic watthour meters and demand registers utilizing microprocessors having a program clock performing timekeeping functions for the register. In particular, the invention relates to calibration of the program clock based on the line frequency of the AC supply.
Since the advent of solid state electronics into the field of AC power consumption measurement, increasingly sophisticated watthour meters and demand registers have utilized programmable microprocessors. These microprocessors accumulate and store usage data, perform involved calculations over programmed time periods and control comprehensive digital displays. Since real time plays an essential role in the function of these electronic watthour meters and demand registers, some means for timekeeping, such as a program clock, is incorporated into the microprocessor.
Most program clocks are maintained by the line frequency of the AC source supplying the load being monitored. The AC line frequency is carefully controlled by the power companies, consequently a clock maintained by this line frequency is sufficiently accurate for the purposes of a demand register. Most power companies provide AC electricity at a line frequency of either 50 Hz or 60 Hz. In the past, separate meters or registers have been provided having a program clock programmable for either a 50 Hz or a 60 Hz signal.
In light of this background, it is an object of the present invention to provide a means for sensing the line frequency and automatically programming the microprocessor program clock accordingly. Another object is to provide a single demand register to accommodate a number of line frequencies without outside intervention.